1. Field of the Invention
The present invention relates to protective structure for an optical fiber coupler and method for protecting the same which splits and couples lights and is utilized in the field of optical communication or optical fiber sensors, etc.
2. Related Background Art
The optical fiber coupler is a device for splitting and coupling lights among a plurality of optical fibers, and is fabricated by contacting coating-removed parts of the plural optical fibers one another and thereafter fusing and elongating the coating-removed bare fiber parts by heating. The fused and elongated region is exposed as bare fiber and is the tapered region where the fiber diameter becomes gradually narrower as going to the center, so that it is required for such region to be protected by a case or other ways.
The conventional protective structure for the optical fiber coupler(Japanese Patent Application: Serial No. 314,350/1991) is shown in FIGS. 1 to 4. In each figure, the region of the coating portion 1a and the coating-removed bare fiber part 1b which is neither fused nor elongated, of an optical fiber 1 are fixed in a case body 3a with adhesive 2 and thereafter an opening part of the case body 3a is closed by a case lid 3b and sealed with adhesive. According to the conventional method, when the coating portion 1a and the bare fiber part 1b of the optical fiber 1 are fixed in the case body 3a with the adhesive 2, the adhesive which has high hardness is used for the coating portion 1a and the bare fiber part 1b to coat the adhesive around the optical fiber in order to strengthen the fixation.
In the conventional protective structure, since the adhesive 2 having high hardness is coated to the circumference of the bare fiber part 1b, in accordance with the change of the outer environment such as the temperature change, the adhesive 2 or the protective case 3 is frequently expanded and/or contracted, and due to this, uneven stress is added to the bare fiber part 1b. This stress results in occurring birefringence in the optical fiber 1 and changing polarization characteristic of the optical fiber coupler. Especially, in the optical fiber coupler using a polarization holding optical fiber, it is a problem that crosstalk, which is an important characteristic parameter of the optical fiber coupler, is degraded.
For the solution to the aforesaid problems, one might consider to lower the hardness of adhesive. However, if the hardness of adhesive is lowered, the stress impressed to the bare fiber part 1b can be lowered but the fixation of the bare fiber part 1b to the protective case 3 is insufficient, therefore it is a problem that characteristic of the optical fiber is fluctuated by vibration or other reasons.